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Alright, I have the cold piston slap which is gradually becoming louder with miles accumulated, no surprize there.

But what I've noticed is that the slap doesn't occur when the engine is started, but rather it develops as heat is built into the cold engine by driving gently, and reaches peak intensity about the time the temp gauge reaches it's "normal" reading. From that point, it slowly diminishes as the engine warms further, and is gone after another 2-5 miles of driving (depending on ambient temp).

Geek type question:
Why the buildup of the slap noise during warmup? I'd think the piston, being the primary recipient of combustion heat during the warmup cycle, would expand faster than the bore during that period and therefore be subject to a snug, bore-fit.

This puzzle rattles in my head at least twice a day during the winter.

That's an interesting question, I wonder if straight 30w (which I wouldn't use in the dead of winter since it's been 10-18°F at night lately here) or a 20w-30 would be quieter from cold start than 10w-30. For that matter, would an engine block heater mean a quieter piston than ambient temp cold startups?

I have thought about if an engine block heater would help. I know when I park my 97OBS in the basement garage where the temps are normally 30-45 degrees rather than 10-20 degrees outside, that the piston slap is not as bad. I would be interested in seeing a post from someone that has had experience with an engine block heater.

I've got semi-syn in mine (tried pre-mix and my own mix) and it doesn't seem to help much.

It's only really noticeable for me when the temp is in the teens or lower (F). And I don't like the noise when it's cold and you step on the gas, takes mine a few minutes after "normal" temp for it to go away and hot heat to actually get going..

On a 93 Wagon, it was the lifters causing prob. Replaced & ok, but the problem returned 2 wks. later. Found a tiny piece of rubber in an oil galley. Hmm?

Second thought...On my 98 OBW, slapping was loud. Asked a guy who works on subies only & he said to put some of this engine cleaner in my oil for a couple of changes. Although the noise is not gone, it did reduce it. SO check with your mechanic. I don't recall the name of mine, but it was a metal can & the color seems to have been blue. Sorry, memory from 9 mos. ago.

I'm not sure it gets cold enough often enough here in Detroit to justify a block heater (the coldest it got during the recent cold snaps was -1F).

The first two winters I ran mobil 1 5W-30. This winter I tried to thicken it up just a bit by mixing (2)qts. 5W-30 with (2)qts. 0W-40, but it didn't seem to make any difference in the noise.

To anyone with a slapper: does the noise continue to get worse every winter (as the mileage accumulates), or does it level off at some point? This is my third winter with this vehicle, and the slap has gotten proggressively louder and longer each winter.

This car is a blast in the snow, it's like a friggin' go-cart. I love dusting Mustangs, Camaros, and 2WD pickups from light to light.

Our 99 outback with ~150k is quieter with amsoil 5w-30 than with either pennzoil 5w-30 or mobil 1 5w-30. Some mornings it doesn't slap at all regardless of temp. On one particular -10*F morning I remember that it sounded fully warmed up as soon as I started it. I think the viscosity and cold flow characteristics of the oil can make a difference.

The piston slapping seems to have leveled off, but the general rattles seem to be getting worse every day. I do notice the same behavior with the engine not becoming completely quiet most days until the coolant temp gauge has been at normal for several miles.

did you ever look into the fact that it might be one of the pulleys? check out www.endwrench.com it talks quite a bit about cold start engine noises. i use Mobil 1 synthetic 15W-50 in all my engines and it greatly improves the engines longevity and smooths engine operation. but i have noticed the knock noise caused by the short piston skirts on our '00 Legacy w/173,000 mi., but after knowing it's normal i don't care. like i said the car has 173K on it.

My 98 O/B has 190,000+kms on it and it's a big-time slapper. It's been between -20 to -35°C here for about a week and you wouldn't believe the noise the thing makes when it first fires up....Neighbors lock their doors and pull the shades...children scatter !!,(Ok..maybe not). It quiets down after about 15 seconds then remains at it's normal clapping level,(which is still quite noisy), until normal operating temperature....then it just makes a loud TAPPING sound only when I accelerate.
When it gets below -30°c I use the block heater and it makes a BIG difference. I've tried every possible viscosity of oil and none of them make much difference.

slaps till it is warm and then quiets down. In the last 20,000 miles it has gotten a bit louder.
SF is a fairly mild climate but on the colder ice on the windows days we had a few days ago it got pretty loud.
I am just running dealer dino.

The other thing on the newer 2.5's that people mistake for piston slap is the timing belt tensioner. The new style tensioner Subaru started using around 1999/2000/2001 (depends on the model) is notorious for going bad, allowing the timing belt to actually slap against the timing belt cover. This also often goes away after warmup, which is why it is often overlooked as piston slap. Mine was particularly bad, and sounded like a bad exhaust manifold gasket.

the other thing that could be causing some tapping noises are the valves. solid lifter Subaru engines are scheduled for shims at 100K. Now if you use synthetic oil the clearance (wear) might never get to the point that the valves will need shimmed.

Originally posted by North Ursalia The other thing on the newer 2.5's that people mistake for piston slap is the timing belt tensioner. The new style tensioner Subaru started using around 1999/2000/2001 (depends on the model) is notorious for going bad, allowing the timing belt to actually slap against the timing belt cover. This also often goes away after warmup, which is why it is often overlooked as piston slap. Mine was particularly bad, and sounded like a bad exhaust manifold gasket.

Ahh... this was a concern of mine. I'm aware of the tensioner noise thing but wasn't sure if there was any way to positively differentiate it from the piston noise.

I would've thought for sure that the tensioner would begin making noise from the get-go rather than needing a short drive for the noise to develop (as on my car), but maybe I'm wrong?

Originally posted by Dr.Outback the other thing that could be causing some tapping noises are the valves. solid lifter Subaru engines are scheduled for shims at 100K. Now if you use synthetic oil the clearance (wear) might never get to the point that the valves will need shimmed.

My '02 uses the old fashioned screw and locknut type adjustment (no shims). I had the covers off at 20k miles to check the clearance and found it had opened by only .001 over spec. I adjusted 'em back down anyway since I was already in there.

and having heard a heck of a lot of piston slap mine is nothing but that.
I have new belts, tensioner, water pump, and the usual seals.
Pistons will slap for a heck of a long time with little effect other than on your nerves.
Any of you folks who have done low money engine jobs like honeing out your cylinders with new rings and bearings when you should have bored it and fitted new pistons know what I am talking about.
When I was in school this was the only way I could afford to run a car.
Now here I am at 53 and I get piston slap again.

Originally posted by cookie and having heard a heck of a lot of piston slap mine is nothing but that.I have new belts, tensioner, water pump, and the usual seals.Pistons will slap for a heck of a long time with little effect other than on your nerves.Any of you folks who have done low money engine jobs like honeing out your cylinders with new rings and bearings when you should have bored it and fitted new pistons know what I am talking about.When I was in school this was the only way I could afford to run a car.

The "cheap" way around that was the knurl tool for the piston skirts to expand little indentations and bumps. It could buy another 30-40k miles of driving on the old pistons.

Now here I am at 53 and I get piston slap again.

It's interesting to see all these problems that automakers had pretty much worked-out long ago popping up again in the quest for emissions and efficiency. e.g. sludging, piston slap, lean stumble, etc.

well when I had the money for that. They used to charge $10 per piston and a few times that was more than I had after buying rings, gaskets, bearings, and doing the valves myself using junkyard parts when mine were too far gone.
Most of those engines made about 30,000 miles before they were so shot that oil consumption was about the same as fuel.
But by then in Maine the body was a pile of rust anyway.